The tenascins are a family of large extracellular glycoproteins designated tenascin-C (cytotactin, TNC*), tenascin-X (TNX), tenascin-R (restrictin, TNR), and tenascin-W (TNW). Their functions in vivo are largely unknown, despite a large literature describing their myriad effects in vitro 1. We recently demonstrated an association of TNX-deficiency with the Ehlers-Danlos syndrome (EDS) 2, 3, a disorder of collagen metabolism causing hyperextensible skin and joints 4,5. Gene targeting studies showed that TNX regulates collagen deposition 6, probably through direct interaction with the collagen fibril. TNC is also collagen fibril-associated and preliminary studies show that TNC can correct the cellular defect in TNX-deficient fibroblasts, suggesting functional redundancy. TNX and/or TNC are induced in fibrotic conditions of heart, liver, skin and lung 1, leading us to wonder whether deficiency of these proteins might restrain fibrosis. Bleomycin-induced pulmonary fibrosis, a widely employed model of lung injury, is of particular interest because TNC is highly induced while TNX is down-regulated 7. Preliminary studies show that Tnc null mice are protected from bleomycin-induced lung injury and fibrosis, while Tnx null mice have greater early mortality, but less late fibrosis. We hypothesize that TNC interacts with known epithelial and mesenchymal integrins, thereby modulating the expression of inflammatory chemokines by alveolar epithelium and facilitating TGF-beta-dependent myofibroblast activation and fibrosis. In contrast to bleomycin lung injury in mice, TNX is induced in humans with pulmonary fibrosis. We hypothesize that TNX over-expression will increase collagen accumulation occurring after bleomycin-induced lung injury.